Control Method and Control Device for Exhaust Gas Control Apparatus

ABSTRACT

It is an object of the invention to provide a technology for appropriately recovering a decreased HC oxidizing ability of rhodium (Rh), in an exhaust gas control apparatus including a catalyst that contains rhodium (Rh) and a particulate filter ( 5 ). In this exhaust gas control apparatus, rich-spike control is prohibited and a NOx storage reduction catalyst is placed in a reduction atmosphere during a period in which a temperature of the NOx storage reduction catalyst is equal to or higher than a predetermined temperature, in a course of decreasing the temperature of the NOx storage reduction catalyst after a PM trapping ability forcible recovery process of the particulate filter ( 5 ) is completed. Thus, the decreased HC oxidizing ability of rhodium (Rh) is recovered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a control method and control device for anexhaust gas control apparatus including a catalyst that contains rhodium(Rh), and a particulate filter.

2. Description of the Related Art

In an exhaust gas control apparatus for an internal combustion engine,which is mounted in, for example, a vehicle, a catalyst that containsplatinum (Pt) deteriorates with use. With the aim of addressing such aproblem, there is a known method in which, when the catalystdeteriorates, the catalyst is placed in a rich atmosphere for apredetermined period, whereby the deteriorated catalyst is recovered. Atechnology related to such a method is disclosed in, for example,Japanese Utility Model Application Publication No. 63-128221.

As an exhaust gas control apparatus for a compression ignition internalcombustion engine (i.e., diesel engine), an exhaust gas controlapparatus is known which is formed by integrally or separately arranginga catalyst that contains rhodium (Rh) and a particulate filter fortrapping particulate matter (hereinafter, referred to as “PM”) in anexhaust system.

In this type of exhaust gas control apparatus, while the PM trappingability of the particulate filter is recovered, the catalyst is exposedto a high-temperature and lean atmosphere. If the catalyst that containsrhodium (Rh) is exposed to the high-temperature and lean atmosphere,rhodium (Rh) moves to the inside of a catalyst carrier, resulting in adecrease in the NOx reducing ability of the catalyst.

Such a decreased NOx reducing ability of the catalyst is recovered, whenthe catalyst is exposed to a rich atmosphere at a high temperature of400° C. or higher. However, there is a problem that, since thetemperature of the exhaust gas released from the compression ignitioninternal combustion engine is low, increasing the temperature of thecatalyst to be 400° C. or higher decreases fuel efficiency.

SUMMARY OF THE INVENTION

The invention is made in light of the above-mentioned circumstances. Itis therefore an object of the invention to provide a control method andcontrol device which can appropriately recover a decreased reducingability of a catalyst in an exhaust gas control apparatus for aninternal combustion engine, the exhaust gas control apparatus beingformed by integrally or separately arranging the catalyst that containsrhodium (Rh) and a particulate filter in an exhaust system of theinternal combustion engine.

According to an aspect of the invention, there is provided a controlmethod for an exhaust gas control apparatus formed by integrally orseparately arranging a catalyst that contains rhodium (Rh) and aparticulate filter in an exhaust system of an internal combustionengine, characterized in that the catalyst is placed in a reductionatmosphere in a course of decreasing a catalyst temperature after a PMtrapping ability forcible recovery process for the particulate filter iscompleted.

In order to recover the PM trapping ability of the particulate filter,the temperature of the particulate filter is increased by forciblyincreasing the temperature of exhaust gas and/or forcibly increasing theamount of reaction heat in the catalyst. Thus, the so-called PM trappingability forcible recovery process is performed so as to oxidize andremove the PM trapped in the particulate filter.

When the PM trapping ability forcible recovery process is performed, thecatalyst is exposed to a high-temperature and lean atmosphere togetherwith the particulate filter. Accordingly, rhodium (Rh) moves to theinside of a catalyst carrier. If rhodium (Rh) moves to the inside of thecatalyst carrier, the reducing ability of the catalyst is decreased.

When the catalyst is exposed to the reduction atmosphere at a hightemperature, the rhodium (Rh), which has moved to the inside of thecatalyst carrier, outcrops to a surface of the catalyst carrier.

Note that, if the temperature of the catalyst is forcibly increased onlyin order to recover the decreased reducing ability of the catalyst, thefuel efficiency may be considerably decreased.

In order to address such a problem, the catalyst is placed in thereduction atmosphere in a period, in which the temperature of thecatalyst has been sufficiently decreased and re-heating of the catalystneed not be performed, in the course of decreasing the catalysttemperature after the PM trapping ability forcible recovery process iscompleted. Thus, the decreased the reducing ability of the catalyst canbe recovered by using heat obtained during the PM trapping abilityforcible recovery process. As a result, an extra temperature increasingprocess for recovering the decreased reducing ability of the catalystneed not be performed, and a decrease in the fuel efficiency issuppressed.

The decreased reducing ability of the catalyst is appropriatelyrecovered, when the catalyst is exposed to the reduction atmosphere at ahigh temperature of approximately 400° C. or higher. Accordingly, thecatalyst may be placed in the reduction atmosphere in a period, in whichthe catalyst temperature is approximately 400° C. or higher, in thecourse of decreasing the catalyst temperature after the PM trappingability forcible recovery process is completed. In this case, an amountof reducing agent required to generate the reduction atmosphere can beminimized.

In the invention, a NOx storage reduction catalyst may be used as thecatalyst that contains rhodium (Rh). Since the NOx storage ability ofthe NOx storage reduction catalyst is limited, the NOx storage abilityneeds to be recovered when required, in the exhaust gas controlapparatus including the NOx storage reduction catalyst.

As a method for recovering the NOx storage ability of the NOx storagereduction catalyst, so-called rich-spike control is effective. In therich-spike control, an air-fuel ratio of the exhaust gas flowing intothe catalyst is made rich by supplying a reducing agent into the exhaustgas flowing upstream of the catalyst.

In the exhaust gas control apparatus including the particulate filterand the NOx storage reduction catalyst, the rich-spike control may beperformed after the PM trapping ability forcible recovery process forthe particulate filter is completed.

If the rich-spike control is performed when the NOx reducing ability ofthe catalyst has been decreased, although the NOx stored in the NOxstorage reduction catalyst is released, the released NOx cannot bereduced sufficiently. Accordingly, the NOx may be released into the airwithout being reduced. In addition, with an increase in the amount ofNOx that has not been reduced, the amount of reducing agent that has notreacted with NOx may increase.

Meanwhile, if the rich-spike control is performed after the PM trappingability forcible recovery process is completed, the catalyst is exposedto the high-temperature and rich atmosphere. Accordingly, the decreasedNOx reducing ability of the catalyst may be recovered. In the rich-spikecontrol, however, the air-fuel ratio of the exhaust gas is made richintermittently, and the length of each period in which the air-fuelratio of the exhaust gas is rich is relatively short. It is thereforedifficult to sufficiently recover the decreased the NOx reducing abilityof the catalyst. Further, the conventional type of rich-spike control isperformed without the characteristics of rhodium (Rh) taken intoconsideration. Accordingly, the catalyst is not always placed in therich atmosphere when the temperature of the catalyst in an appropriatetemperature range.

According to the invention, if the catalyst that contains rhodium (Rh)is the NOx storage reduction catalyst, the rich-spike control may beprohibited after the PM trapping ability forcible recovery process iscompleted, whereby the catalyst is placed in the reduction atmosphere.

When the catalyst is placed in the reduction atmosphere, preferably anair-fuel ratio of the exhaust gas is made higher than that when the richatmosphere is generated by the rich-spike control for the followingreason.

If the rich atmosphere similar to that generated by the rich-spikecontrol is formed when the NOx reducing ability of the catalyst has beendecreased, a relatively large amount of NOx is released from the NOxstorage reduction catalyst, and therefore an amount of NOx released intothe air without being reduced may increase.

Examples of a method for placing the catalyst in the reductionatmosphere include a method in which a small amount of reducing agent issupplied to the exhaust gas at intervals shorter than those in therich-spike control, and a method in which an air-fuel ratio in theinternal combustion engine is made low.

In the exhaust gas control apparatus including the NOx storage reductioncatalyst and the particulate filter, a process for recovering the NOxstorage reduction catalyst from sulfur poisoning (hereinafter, referredto as a “sulfur poisoning recovery process for the NOx storage reductioncatalyst”) may be performed subsequent to the PM trapping abilityforcible recovery process.

The control according to the invention is different from the sulfurpoisoning recovery process in the following point. In the controlaccording to the invention, the catalyst is placed in the reductionatmosphere without forcibly increasing the temperature of the catalystand without forcibly maintaining the temperature of the catalyst. Incontrast to this, in the sulfur poisoning recovery process, the catalystis placed in the reduction atmosphere while the temperature of thecatalyst is forcibly increased and maintained.

If the sulfur poisoning recovery process is performed, the catalyst isexposed to the high-temperature and rich atmosphere. Therefore thedecreased NOx reducing ability of the catalyst can be recovered.

Accordingly, when the sulfur poisoning recovery process is performedsubsequent to the PM trapping ability forcible recovery process, thecontrol according to the invention is prohibited. On the other hand,when the sulfur poisoning recovery process is not performed subsequentto the PM trapping ability forcible recovery process, the controlaccording to the invention is performed. In this case, the catalyst isprevented from unnecessarily being placed in the reduction atmosphere,and therefore the fuel efficiency is prevented from being decreased.

According to another aspect of the invention, there is provided acontrol device for an exhaust gas control apparatus including aparticulate filter provided in an exhaust system of an internalcombustion engine, and a catalyst that is provided integrally with orseparately from the particulate filter in the exhaust system and thatcontains rhodium, the control device being characterized by includingrecovery means for increasing a temperature of the particulate filterand a temperature of the catalyst, thereby forcibly recovering a PMtrapping ability of the particulate filter; and NOx reducing abilityrecovery means for placing the catalyst in a reduction atmosphere in acourse of decreasing the temperature of the catalyst after the PMtrapping ability of the particulate filter is forcibly recovered.

It is to be understood that “storage” used herein means retention of asubstance (solid, liquid, gas molecules) in the form of at least one ofadsorption, adhesion, absorption, trapping, occlusion, and others.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned embodiment and other embodiments, objects, features,advantages, technical and industrial significance of this invention willbe better understood by reading the following detailed description ofthe exemplary embodiments of the invention, when considered inconnection with the accompanying drawings, in which:

FIG. 1 is a view schematically showing a structure of an internalcombustion engine to which the invention is applied;

FIG. 2 is a graph showing a temperature at which a NOx reducing abilityof a NOx storage reduction catalyst is activated;

FIG. 3 is a flowchart showing a routine of catalyst's NOx reducingability recovery control; and

FIG. 4 is a graph showing a concrete method for performing an exhaustgas enriching process.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the following description, the invention will be described in moredetail in terms of exemplary embodiments.

An internal combustion engine 1 shown in FIG. 1 is a compressionignition internal combustion engine (i.e., diesel engine). The internalcombustion engine 1 is provided with an intake passage 2 and an exhaustpassage 3. An intake throttle valve 4 is provided in the intake passage2. A particulate filter 5 is provided in the exhaust passage 3. Theparticulate filter 5 supports a NOx storage reduction catalyst thatcontains rhodium (Rh).

A reducing agent supply valve 6, which injects fuel from the internalcombustion engine 1 as a reducing agent, is provided in the exhaustpassage 3 at a position upstream of the particulate filter 5. An exhaustgas temperature sensor 7 is provided in the exhaust passage 3 at aposition downstream of the particulate filter 5.

An EGR passage 8 permits communication between the intake passage 2 andthe exhaust passage 3. An EGR valve 9 is provided in the EGR passage 8.

Each of the intake throttle valve 4, the reducing agent supply valve 6,the exhaust gas temperature sensor 7, and the EGR valve 9 iselectrically connected to an ECU 10.

The ECU 10 performs known controls such as fuel injection control andEGR control based on operation states of the exhaust gas temperaturesensor 7 and the internal combustion engine 1. The ECU 10 also performscatalyst's NOx reducing ability recovery control that is a main featureof the invention. Hereafter, the catalyst's NOx reducing abilityrecovery control will be described in detail.

Since the PM trapping ability of the particulate filter 5 is limited,the ECU 10 performs the PM trapping ability forcible recovery processbefore the limit of the PM trapping ability is reached. In the PMtrapping ability forcible recovery process, the ECU 10 increases thetemperature of the exhaust gas and/or increases the amount of reactionheat in the NOx storage reduction catalyst by performing post-injectionand/or supplying fuel from the reducing agent supply valve 6 into theexhaust gas, thereby forcibly increasing the temperature of theparticulate filter 5.

When the PM trapping ability forcible recovery process for theparticulate filter 5 is performed, the NOx storage reduction catalystsupported by the particulate filter 5 is also exposed to thehigh-temperature and rich atmosphere. At this time, rhodium (Rh)contained in the NOx storage reduction catalyst moves to the inside ofthe catalyst carrier. As a result, the NOx reducing ability of the NOxstorage reduction catalyst (especially, a hydrocarbon (HC) oxidizingability) is decreased.

If the HC oxidizing ability of the NOx storage reduction catalyst isdecreased, the NOx reducing ability of the NOx storage reductioncatalyst is decreased. Namely, if the HC oxidizing ability of the NOxstorage reduction catalyst is decreased, when the NOx storage ability ofthe NOx storage reduction catalyst is recovered, that is, when the richspike control, in which fuel (hydrocarbon (HC)) is intermittentlysupplied from the reducing agent supply valve 6 into the exhaust gas, isperformed, it becomes difficult for the hydrocarbon (HC) to transforminto a reaction activated substance in the NOx storage reductioncatalyst. Accordingly, the NOx released from the NOx storage reductioncatalyst may be released into the air without being reduced, and thehydrocarbon supplied to the NOx storage reduction catalyst may bereleased into the air without reacting with NOx.

FIG. 2 is a graph showing the temperature at which the NOx reducingability of the NOx storage reduction catalyst is activated. Before thePM trapping ability forcible recovery process for the particulate filter5 is performed, the NOx reducing ability of the NOx storage reductioncatalyst is activated at a temperature of approximately 300° C. Incontrast to this, after the PM trapping ability forcible recoveryprocess is performed, the NOx reducing ability of the NOx storagereduction catalyst is not activated until the temperature increases tobe approximately 350° C. or higher.

The temperature of the exhaust gas released from the compressionignition internal combustion engine is approximately 300° C. at timesother than a period in which the internal combustion engine is operatedat high load. As described above, an increase in the temperature, atwhich the NOx reducing ability is activated, increases a possibilitythat the amounts of NOx and HC released into the air increase when therich-spike control is performed.

Accordingly, when the PM trapping ability forcible recovery process isperformed, the decreased HC oxidizing ability of the NOx storagereduction catalyst needs to be recovered. In order to recover thedecreased HC oxidizing ability, rhodium (Rh), which has moved to theinside of the catalyst carrier, needs to outcrop to the surface of thecatalyst carrier again.

Rhodium (Rh), which has moved to the inside of the catalyst carrier,outcrops to the surface of the catalyst carrier, when the catalyst isexposed to the reduction atmosphere at a high temperature of 400° C. orhigher. Therefore, if the exhaust gas flowing in the NOx storagereduction catalyst is made rich after the temperature of the NOx storagereduction catalyst is increased to be 400° C. or higher, the decreasedHC oxidizing ability can be recovered.

Examples of an effective method for increasing the temperature of theNOx storage reduction catalyst to be 400° C. or higher, that is, atemperature in a high temperature range, include a method in which thetemperature of the exhaust gas is increased by performing post injectionand a method in which the amount of reaction heat in the NOx storagereduction catalyst is increased by supplying fuel into the exhaust gas.However, there is a problem common to these methods, that is, a decreasein the fuel efficiency.

Accordingly, in the catalyst's NOx reducing ability recovery controlaccording to the embodiment, the particulate filter 5 is placed in thereduction atmosphere (rich atmosphere) in the period in which thetemperature of the NOx storage reduction catalyst is 400° C. or higher,in the course of decreasing the temperature of the catalyst after the PMtrapping ability forcible recovery process is completed.

Hereafter, the catalyst's NOx reducing ability recovery control will bedescribed with reference to FIG. 3. FIG. 3 is a flowchart showing theroutine of the catalyst's NOx reducing ability recovery control. Thecatalyst's NOx reducing ability recovery control routine is stored inROM of the ECU 10 in advance. The catalyst's NOx reducing abilityrecovery control routine is an interrupt routine that is performed bythe ECU 10 when the PM trapping ability forcible recovery process iscompleted.

In the catalyst's NOx reducing ability recovery control routine, the ECU10 initially determines in step S101 whether a PM trapping abilityforcible recovery completion flag shows “1”. The PM trapping abilityforcible recovery completion flag is stored in RAM or the like inadvance. When the PM trapping ability forcible recovery process iscompleted, “1” is stored. When the catalyst's NOx reducing abilityrecovery control is completed, “0” is stored.

When it is determined in step S101 that the PM trapping ability forciblerecovery completion flag shows “0”, the ECU 10 ends the routine. On theother hand, when it is determined in step S101 that the PM trappingability forcible recovery completion flag shows “1”, the ECU 10 thenperforms step S102.

In step S102, the ECU 10 receives a signal Tout which indicates atemperature of the exhaust gas released from the particulate filter 5(hereinafter, referred to as “an outflow exhaust gas temperature Tout”),and which is output from the exhaust gas temperature sensor 7.

In step S103, the ECU 10 determines whether the outflow exhaust gastemperature Tout received in step S102 is equal to or higher than apredetermined temperature Ts (e.g., 400° C.).

When it is determined in step S103 that the outflow exhaust gastemperature Tout is equal to nor higher than the predeterminedtemperature (Tout<Ts), the ECU estimates that a bed temperature of theNOx storage reduction catalyst is lower than the predeterminedtemperature Ts, and then performs step S110. In step S110, the ECU 10changes the value of the PM trapping ability forcible recoverycompletion flag to “0”, and then ends the routine.

On the other hand, when it is determined in step S103 that the outflowexhaust gas temperature Tout is equal to or higher than thepredetermined temperature Ts (Tout≧Ts), the ECU 10 estimates that thebed temperature of the NOx storage reduction catalyst is equal to orhigher than the predetermined temperature T_(s), and then performs stepS104.

In step S104, the ECU 10 prohibits the rich-spike control.

In step S105, the ECU 10 performs an exhaust gas enriching process formaking the exhaust gas flowing into the particulate filter 5 rich. Inthe exhaust gas enriching process, the ECU 10 controls the reducingagent supply valve 6 such that fuel is intermittently supplied into theexhaust gas.

At this time, the ECU 10 controls the reducing agent supply valve 6 suchthat the amount of fuel supplied from the reducing agent supply valve 6during each supply become smaller than that in the rich-spike control,and the interval between the fuel supplies become shorter than that inthe rich-spike control, as shown in FIG. 4.

The amount of fuel supplied from the reducing agent supply valve 6during each supply is made smaller than that in the rich-spike controlfor the following reason. If the same amount of hydrocarbon (HC) as thatin the rich-spike control is supplied to the NOx storage reductioncatalyst when the HC oxidizing ability of the NOx storage reductioncatalyst has been decreased, the amount of NOx released from the NOxstorage reduction catalyst increases, and the amount of NOx releasedinto the air without being reduced also increases.

The amount of fuel supplied from the reducing agent supply valve 6during each supply is made smaller than that in the rich-spike controlalso for the following reason. If the same amount of hydrocarbon (HC) asthat in the rich-spike control is supplied to the NOx storage reductioncatalyst when the HC oxidizing ability of the NOx storage reductioncatalyst has been decreased, the amount of hydrocarbon (HC) that isreleased into the air without reacting with NOx may increase.

The interval between the fuel supplies is made shorter than that in therich-spike control for the following reason. The temperatures of theparticulate filter 5 and the NOx storage reduction catalyst rapidlydecrease after the PM trapping ability forcible recovery process iscompleted. Accordingly, if the fuel is supplied with the same intervalsas those in the rich-spike control, the temperature of the NOx storagereduction catalyst may decrease to be the predetermined temperature Tsor lower, before the HC oxidizing ability is recovered.

In step S106, the ECU 10 receives the signal (i.e., outflow exhaust gastemperature) Tout output from the exhaust gas temperature sensor 7again.

In step S107, the ECU 10 determines whether the outflow exhaust gastemperature Tout received in step S106 has decreased to be lower thanthe predetermined temperature Ts.

When it is determined in step S107 that the outflow exhaust gastemperature Tout has not decreased to be lower than the predeterminedtemperature Ts (Tout≧Ts), the ECU 10 determines that the bed temperatureof the NOx storage reduction catalyst is still equal to or higher thanthe predetermined temperature Ts, and then performs step S105 and thefollowing steps again.

On the other hand, when it is determined in step S107 that the outflowexhaust gas temperature Tout has decreased to be lower than thepredetermined temperature Ts (Tout<Ts), the ECU 10 determines that thebed temperature of the NOx storage reduction catalyst has decreased tobe lower than the predetermined temperature Ts, and then performs stepS108.

In step S108, the ECU 10 ends the exhaust gas enriching process.

In step S109, the ECU 10 removes prohibition of the rich-spike control.

In step S110, the ECU 10 changes the value of the PM trapping abilityforcible recovery process completion flag to “0”.

When the ECU 10 performs the catalyst's NOx reducing ability recoverycontrol routine in the above-mentioned manner, the HC oxidizing abilityof the NOx storage reduction catalyst can be recovered by using the heatobtained during the PM trapping ability forcible recovery process. As aresult, a decrease in the fuel efficiency due to an increase in thetemperature of the NOx storage reduction catalyst can be suppressed.

In the embodiment, the amount of fuel supplied into the exhaust gasduring each supply is made smaller than that in the rich-spike control.Also, in the embodiment, the interval between the fuel supplies is madeshorter than that in the rich-spike control. Accordingly, the HCoxidizing ability of the NOx storage reduction catalyst can be recoveredin the period in which the bed temperature of the NOx storage reductioncatalyst is equal to or higher than the predetermined temperature Ts. Inaddition, the amount of NOx released into the air without being reducedand the amount of hydrocarbon (HC) that are released into the airwithout reacting with NOx can be decreased.

If the exhaust gas enriching process is performed after the PM trappingability forcible recovery process is completed, the bed temperature ofthe NOx storage reduction catalyst may be maintained at a temperatureequal to or higher than the predetermined temperature Ts for a long timedue to the heat generated by reaction of rhodium (Rh) and hydrocarbon(HC). In such a case, any one of the following methods may be employed;(1) the exhaust gas enriching process is completed when the performancetime of the exhaust gas enriching process becomes equal to or longerthan a predetermined time, (2) the temperature of the NOx storagereduction catalyst is gradually decreased by decreasing the fuel supplyamount with an increase in the number of times of fuel supply, and (3)an interval is provided every time fuel supply has been performed apredetermined number of times such that the temperature of the NOxstorage reduction catalyst is decreased in a stepwise manner.

In the embodiment, supplying fuel into the exhaust gas from the reducingagent supply valve 6 is employed as a concrete method for performing theexhaust gas enriching process. However, the air-fuel ratio of theexhaust gas released from the internal combustion engine 1 may bedecreased by increasing the amount of the EGR gas.

In the embodiment, the particulate filter 5 and the NOx storagereduction catalyst are integrally provided in the exhaust passage 3.However, the particulate filter 5 and the NOx storage reduction catalystmay be separately provided in the exhaust passage 3.

For example, the particulate filter 5 and the NOx storage reductioncatalyst may be provided in the exhaust passage 3 in series (preferably,the NOx storage reduction catalyst is provided upstream of theparticulate filter 5). Note that, in this case, the reducing agentsupply valve 6 needs to be provided upstream of the NOx storagereduction catalyst.

Hereafter, the other embodiments will be described.

When the amount of sulfur contained in the fuel used in the internalcombustion engine 1 is large, sulfur poisoning (i.e., S poisoning)occurs in the NOx storage reduction catalyst. Accordingly, the sulfurpoisoning recovery process may be performed subsequent to the PMtrapping ability forcible recovery process.

In the sulfur poisoning recovery process, the catalyst is placed in therich atmosphere while the temperature of the NOx storage reductioncatalyst is maintained at a high temperature. Accordingly, the decreasedHC oxidizing ability of the NOx storage reduction catalyst can berecovered.

Therefore, when performing the sulfur poisoning recovery processsubsequent to the PM trapping ability forcible recovery process, the ECU10 prohibits the catalyst's NOx reducing ability recovery control. Onthe other hand, when not performing the sulfur poisoning recoveryprocess subsequent to the PM trapping ability forcible recovery process,the ECU 10 performs the catalyst's NOx reducing ability recoverycontrol.

In this case, the catalyst's NOx reducing ability recovery control isprevented from being unnecessarily performed. Accordingly, fuelconsumption due to the catalyst's NOx reducing ability recovery controlcan be suppressed.

1. A control method for an exhaust gas control apparatus formed byintegrally or separately arranging a catalyst that contains rhodium anda particulate filter for trapping particulate matter, in an exhaustsystem of an internal combustion engine, comprising: performing aparticulate matter trapping ability forcible recovery process forforcibly recovering a particulate matter trapping ability of theparticulate filter by increasing a temperature of the particulate filterand a temperature of the catalyst, and placing the catalyst in areduction atmosphere in a course of decreasing the temperature of thecatalyst after the particulate matter trapping ability forcible recoveryprocess is completed.
 2. The control method for an exhaust gas controlapparatus according to claim 1, wherein the catalyst is placed in areduction atmosphere in a period in which the temperature of thecatalyst is equal to or higher than a predetermined temperature.
 3. Thecontrol method for an exhaust gas control apparatus according to claim2, wherein the predetermined temperature is approximately 400° C.
 4. Thecontrol method for an exhaust gas control apparatus according to claim1, wherein: the catalyst is a NOx storage reduction catalyst, and aprocess for recovering a NOx storage reduction ability of the catalystis prohibited from being performed, when the catalyst is placed in thereduction atmosphere.
 5. The control method for an exhaust gas controlapparatus according to claim 4, wherein an air-fuel ratio of exhaust gaswhen the catalyst is placed in the reduction atmosphere is made higherthan an air-fuel ratio of the exhaust gas during the process forrecovering the NOx storage reduction ability of the catalyst.
 6. Acontrol device for an exhaust gas control apparatus, comprising:recovery portion that forcibly recovers a particulate matter trappingability of a particulate filter that is provided in an exhaust system ofan internal combustion engine by increasing a temperature of theparticulate filter and a temperature of a catalyst that is providedintegrally with or separately from the particulate filter in the exhaustsystem and that contains rhodium; and NOx reducing ability recoveryportion that places the catalyst in a reduction atmosphere in a courseof decreasing the temperature of the catalyst after the particulatematter trapping ability of the particulate filter has been forciblyrecovered.
 7. The control method for an exhaust gas control apparatusaccording to claim 2, wherein: the catalyst is a NOx storage reductioncatalyst, and a process for recovering a NOx storage reduction abilityof the catalyst is prohibited from being performed, when the catalyst isplaced in the reduction atmosphere.
 8. The control method for an exhaustgas control apparatus according to claim 3, wherein: the catalyst is aNOx storage reduction catalyst, and a process for recovering a NOxstorage reduction ability of the catalyst is prohibited from beingperformed, when the catalyst is placed in the reduction atmosphere. 9.The control method for an exhaust gas control apparatus according toclaim 7, wherein an air-fuel ratio of exhaust gas when the catalyst isplaced in the reduction atmosphere is made higher than an air-fuel ratioof the exhaust gas during the process for recovering the NOx storagereduction ability of the catalyst.
 10. The control method for an exhaustgas control apparatus according to claim 8, wherein an air-fuel ratio ofexhaust gas when the catalyst is placed in the reduction atmosphere ismade higher than an air-fuel ratio of the exhaust gas during the processfor recovering the NOx storage reduction ability of the catalyst.